A Custom API starts as either an extension of an existing Extract API (allowing you to override or correct data returned from it), or as a blank canvas API with which rules can be given.
To create or edit a Custom API, go to the Diffbot Dashboard and select "Custom APIs" on the left navigation. Here you can view a list of APIs you have created. Click an existing API to edit it, or click "Create New" on the top right hand corner of the screen to create a new one.
When creating an API you will see this screen:
Select an existing Extract API to extend, or select "Custom" to start with a blank canvas.
Your new API will be ready immediately, either for individual calls, your own app integration, or from within Crawl or Bulk APIs.